As an example of the prior art infrared lenses capable of producing excellent images and sturdy enough to endure severe environments, an infrared optical system suitable for use in surveillance cameras has been proposed which is compatible with infrared rays through far infrared rays, namely, with a wavelength range from 3 μm to 14 μm, and is of dual-lens configuration where the first lens disposed closer to an object is a convex meniscus lens having its convex surface faced to the object while the second lens disposed closer to the image plane is another convex meniscus lens having its concave surface faced to the object, and at least one of the first and second lenses has its opposite surfaces processed to serve as diffraction optics (See Patent Document 1 or Official Gazette of JP-A-2010-113191).
The infrared optical system disclosed in Patent Document 1 is substantially inappropriate to use for a wide-angle lens since its first lens is the convex meniscus lens. In embodiments in Patent Document 1, all the lenses are made of chalcogenide. Chalcogenide is low in diffractive index and great in chromatic dispersion, and hence, in order to compensate for chromatic aberration, the lens must have its surface(s) processed to serve as diffraction optics. In Patent Document 1, all the embodiments have their respective lens surfaces processed to be diffraction optics.
The present invention is made to overcome the aforementioned disadvantages of the prior art infrared lenses, and accordingly, it is an object of the present invention to provide an infrared fixed-focus lens that is of wide-angle, is made of germanium exhibiting a low chromatic dispersion, and includes no lens pieces with a surface serving as diffraction optics.